1. Field of the Invention
The present invention relates to a motor control system and a vacuum pump equipped with the motor control system, and more particularly to a motor control system capable of shortening the starting time of a turbo molecular pump and a vacuum pump equipped with the motor control system.
2. Description of the Related Art
As a result of recent developments in electronics, there is a rapidly increasing demand for semiconductor devices, such as memories and integrated circuits.
Such semiconductor devices are manufactured by doping semiconductor substrates of a very high purity with impurities to impart electrical properties thereto, by stacking together semiconductor substrates with minute circuit patterns formed thereon, etc.
In order to avoid the influences of dust in the air, etc., such operations must be conducted in a chamber in a high vacuum state. To evacuate the chamber, a vacuum pump is generally used; in particular, a turbo molecular pump, which is a kind of vacuum pump, is widely used since it involves little residual gas, allows maintenance with ease, etc. Further, a semiconductor manufacturing process involves a number of steps in which various process gasses are caused to act on a semiconductor substrate, and, the turbo molecular pump is used not only to create a vacuum state in the chamber but also to evacuate such process gases from the chamber.
Further, in electron microscope equipment, a turbo molecular pump is used to create a high vacuum state in the chamber of the electron microscope in order to prevent refraction of the electron beam, etc. due to the presence of dust or the like.
Further, a turbo molecular pump is used in a movable simple vacuum chamber, or in order to place a flat panel display manufacturing apparatus in a vacuum state.
Such a turbo molecular pump is composed of a turbo molecular pump main body 100 for sucking and evacuating gas from the chamber of a semiconductor manufacturing apparatus or the like, and a control device 200 for controlling the turbo molecular pump main body.
FIG. 7 shows the construction of a turbo molecular pump.
In FIG. 7, the turbo molecular pump main body 100 has an inlet port 101 formed at the upper end of a round outer cylinder 127. On the inner side of the outer cylinder 127, there is provided a rotary member 103 in the periphery of which there are formed radially and in a number of stages a plurality of rotary vanes 102a, 102b, 102c, . . . consisting of turbine blades for sucking and evacuating gases.
Mounted at the center of this rotary member 103 is a rotor shaft 113, which is floatingly supported and position-controlled by, for example, a 5-axis control magnetic bearing.
An upper radial electromagnet 104 consists of four electromagnets arranged in pairs in the X- and Y-axes. In close proximity to and in correspondence with the upper radial electromagnet 104, there is provided an upper radial sensor 107 consisting of four electromagnets. The upper radial sensor 107 detects a radial displacement of the rotor shaft 113, and transmits a displacement signal to the control device 200.
In the control device 200, the upper radial electromagnet 104 is excitation-controlled through a compensation circuit with a PID adjustment function (not shown in the drawings) based on the displacement signal obtained through detection by the upper radial sensor 107, thus adjusting the upper radial position of the rotor shaft 113.
The rotor shaft 113 is formed of a high magnetic-permeability material (such as iron), and is attracted by the magnetic force of the upper radial electromagnet 104. This adjustment is conducted independently in the X-axis direction and the Y-axis direction.
Further, a lower radial electromagnet 105 and a lower radial sensor 108 are arranged in the same way as the upper radial electromagnet 104 and the upper radial sensor 107, adjusting the lower radial position of the rotor shaft 113 in the same manner as the upper radial position thereof.
Further, axial electromagnets 106A and 106B are arranged so as to sandwich from above and below a circular metal disc 111 provided in the lower portion of the rotor shaft 113. The metal disc 111 is formed of a high magnetic-permeability material, such as iron.
Further, below the rotor shaft 113, there is provided an axial sensor 109 for detecting an axial displacement signal of the rotor shaft 113. An axial displacement obtained through detection by the axial sensor 109 is transmitted to the control device 200.
Based on the displacement signal obtained through detection by the axial sensor 109, the control device 200 excitation-controls the axial electromagnets 106A and 106B. At this time, the axial electromagnet 106A attracts the metal disc 111 upwardly by magnetic force, and the axial electromagnet 106B attracts the metal disc 111 downwardly.
In this way, the magnetic bearing appropriately adjusts the magnetic force applied to the rotor shaft 113, thereby magnetically levitating the rotor shaft 113 and retaining it in a non-contact fashion.
Further, there is provided a motor 121, which is a so-called brush-less motor. The motor 121 is equipped with an RPM detecting sensor, a motor current detecting sensor, a motor temperature detecting sensor, etc. described below, and, on the basis of detection signals from these sensors, the RPM, etc. of the rotor shaft 113 are controlled by the control device 200. The construction of the control system for the motor 121 will be described in detail below.
Formed on the rotor shaft 113 are the rotary vanes 102a, 102b, 102c, . . . . There are arranged a plurality of stationary vanes 123a, 123b, 123c, . . . , with a slight gap being between them and the rotary vanes 102a, 102b, 102c, . . . . Further, in order to downwardly transfer through collision the molecules of the exhaust gas, the rotary vanes 102a, 102b, 102c, . . . are respectively inclined by a predetermined angle with respect to planes perpendicular to the axis of the rotor shaft 113. Similarly, the stationary vanes 123 are respectively inclined by a predetermined angle with respect to planes perpendicular to the axis of the rotor shaft 113, and are arranged so as to protrude toward the interior of the outer cylinder 127 and in alternate stages with the rotary vanes 102.
Further, one ends of the stationary vanes 123 are supported while being inserted into recesses between a plurality of stationary vane spacers 125a, 125b, 125c, . . . stacked together. The stationary vane spacers 125 are ring-like members formed of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing such metals as components.
Further, in the outer periphery of the stationary vane spacers 125, the outer cylinder 127 is secured in position with a slight gap therebetween. At the bottom of the outer cylinder 127, there is arranged a base portion 129, and, between the stationary vane spacers 125 and the base portion 129, there is arranged a threaded spacer 131. In the portion of the base 129 which is below the threaded spacer 131, there is formed an exhaust port 133, which communicates with the exterior.
The threaded spacer 131 is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or an alloy containing such metals as components, and has on the inner peripheral surface thereof a plurality of spiral thread grooves 131a. The direction of the spiral thread grooves 131a is determined such that, when the molecules of the exhaust gas move in the rotating direction of the rotary member 103, these molecules are transferred toward the exhaust port 133.
Further, in the lowermost portion of the rotary member 103 connected to the rotary vanes 102a, 102b, 102c, . . . , there is provided a rotary vane 102d, which extends vertically downwards. The outer peripheral surface of the rotary vane 102d, is cylindrical, and protrudes toward the inner peripheral surface of the threaded spacer 131 so as to be in close proximity to the threaded spacer 131 with a predetermined gap therebetween.
Further, the base portion 129 is a disc-like member constituting the base portion of the turbo molecular pump main body 100, and is generally formed of a metal, such as iron, aluminum, or stainless steel. The base portion 129 physically retains the turbo molecular pump main body 100, and also functions as a heat conduction path, so that it is desirable to use a metal that is rigid and of high heat conductivity, such as iron, aluminum, or copper, for the turbo molecular pump main body 100.
When, in this construction, the rotor shaft 113 is driven by the motor 121 and rotates together with the rotary vanes 102, an exhaust gas from a chamber is sucked in through the inlet port 101 by the action of the rotary vanes 102 and the stationary vanes 123.
Then, the exhaust gas sucked in through the inlet port 101 flows between the rotary vanes 102 and the stationary vanes 123, and is transferred to the base portion 129. At this time, the temperature of the rotary vanes 102 rises due to the friction heat generated when the exhaust gas comes into contact with the rotary vanes 102, conduction of the heat generated in the motor 121, etc, and this heat is transmitted to the stationary vanes 123 side by radiation or conduction due to the gas molecules, etc. of the exhaust gas. Further, the stationary vane spacers 125 are bonded together in the outer periphery, and transmit to the exterior the heat received by the stationary vanes 123 from the rotary vanes 102, the friction heat generated when the exhaust gas comes into contact with the stationary vanes 123, etc.
The exhaust gas transferred to the base portion 129 is sent to the exhaust port 133 while being guided by the thread grooves 131a of the threaded spacer 131.
In the above-described example the threaded spacer 131 is arranged in the outer periphery of the rotary vane 102d, and the thread grooves 131a are formed in the inner peripheral surface of the threaded spacer 131. However, conversely to the above, the thread grooves may be formed in the outer peripheral surfaces of the rotary vane 102d, and a spacer with a cylindrical inner peripheral surface may be arranged in the periphery thereof.
Further, in order that the gas sucked in through the inlet port 101 may not enter the electrical section formed by the motor 121, the lower radial electromagnet 105, the lower radial sensor 108, the upper radial electromagnet 104, the upper radial sensor 107, etc., the periphery of the electrical section is covered with a stator column 122, and a predetermined pressure is maintained in the interior of the electrical section with a purge gas.
For this purpose, piping (not shown in the drawings) is arranged in the base portion 129, and the purge gas is introduced through the piping. The purge gas thus introduced flows through the gaps between a protective bearing 120 and the rotor shaft 113, between the rotor and stator of the motor 121, and between the stator column 122 and the rotary vanes 102 before being transmitted to the exhaust port 133.
Here, the turbo molecular pump main body 100 requires control based on individually adjusted specific parameters (e.g., the specification of the model and the properties corresponding to the model). To store these control parameters, the turbo molecular pump main body 100 has an electronic circuit portion 141.
The electronic circuit portion 141 is formed by a semiconductor memory, such as EEP-ROM, an electronic component for the access thereto, such as a semiconductor device, a substrate 143 for the mounting thereof, etc. The electronic circuit portion 141 is accommodated in the lower portion near the center of the base portion 129 constituting the lower portion of the turbo molecular pump main body 100, and is closed by a hermetic bottom cover 145.
Incidentally, for enhanced reactivity, the process gas may be introduced into the chamber in a high temperature state. When it reaches a certain temperature by being cooled at the time of evacuation, such process gas may be solidified to precipitate a product in the exhaust system. Then, when such process gas is cooled and solidified in the turbo molecular pump main body 100, it adheres to the inner wall of the turbo molecular pump main body 100 and is deposited thereon. For example, when SiCl4 is used as the process gas in an Al etching apparatus, a solid product (e.g., AlCl3) is precipitated when the apparatus is in a low vacuum state (760 [torr] to 10−2 [torr]) and at lower temperature (approximately 20[° C.]), and adheres to and is deposited on the inner wall of the turbo molecular pump main body 100 as can be seen from a vapor pressure curve.
When precipitate of the process gas is deposited inside the turbo molecular pump main body 100, the deposit narrows the pump flow path, which leads to a deterioration in the performance of the turbo molecular pump main body 100. For example, the above-mentioned product is likely to solidify and adhere to the portion near the exhaust port, in particular, near the rotary vanes 102 and the threaded spacer 131, where the temperature is low.
To solve this problem, there has been conventionally adopted a control system (hereinafter referred to as TMS (temperature management system)), in which a heater (not shown in the drawings), an annular water cooling tube 149, etc. are wound around the outer periphery of the base portion 129 or the like, and in which a temperature sensor (e.g., a thermistor) (not shown in the drawings) is embedded, for example, in the base portion 129, the heating by the heater and the cooling by the water cooling tube 149 being controlled based on a signal from the temperature sensor so as to maintain the base portion 129 at a fixed, high temperature (set temperature).
Here, a conventional motor control system will be described. FIG. 8 shows the construction of the conventional motor control system.
In FIG. 8, a motor control system 300 is equipped with the motor 121 on the turbo molecular pump main body 100 side.
Further, the motor 121 is equipped with an RPM detecting sensor 124 on the stator side thereof. The RPM detecting sensor 124 is arranged so as to surround the rotor shaft 113, and consists, for example, of a semiconductor Hall sensor. The RPM detecting sensor 124 detects the rotating magnetic flux density of the motor 121, thereby detecting the RPM of the rotor shaft 113.
Further, the motor 121 has on the stator side thereof three-phase motor windings 126U, 126V, and 126W. These motor windings 126U, 126V, and 126W are also arranged so as to surround the rotor shaft 113. Further, the motor windings 126U, 126V, and 126W are equipped with motor current detecting sensors 128 (only one of which is shown in the drawing), and the motor current detecting sensors 128 detect motor current Im flowing through the motor windings 126U, 126V, and 126W.
Further, the motor windings 126U, 126V, and 126W are connected to a motor driving circuit 220 on the control device 200 side.
A DC voltage is supplied to the motor driving circuit 220 from a power source 230 (In the drawing, the + side will be referred to as a positive pole 230a, and the − side will be referred to as a negative pole 230b). The motor driving circuit 220 is equipped with inverter circuits 222 (only one of which is shown in the drawing) respectively corresponding to the motor windings 126U, 126V, and 126W, and power is supplied to the motor windings 126U, 126V, and 126W through the inverter circuits 222. Each of these inverter circuits 222 is composed, for example, of two transistors 222a and 222b for one motor winding 126U.
Further, a drive signal is input to the motor driving circuit 220 from a drive control circuit 210. By this drive signal, the power supplied from the inverter circuits 222 to the motor windings 126U, 126V, and 126W is controlled. FIG. 9 is a block diagram showing the drive control circuit.
In FIG. 9, input to the drive control circuit 210 are detection signals from the RPM detecting sensor 124 and the motor current detecting sensors 128. These detection signals are input to a comparator 212.
Further, a command signal is input to the comparator 212 from a reference value setting circuit 214, and this command signal is a signal indicating, for example, a reference RPM.
The comparator 212 compares, for example, the reference RPM indicated by the command signal with the detection signal from the RPM detecting sensor 124 to effect PID compensation, and, further, compares this output signal as a current command value with the detection signals from the motor current detecting sensors 128 to effect PID compensation. Thereafter, the comparator 212 outputs this output signal to a PWM control circuit 216.
The comparator 212 is equipped with a current limiter circuit (not shown in the drawings), and this current limiter circuit performs control based on the comparison result output to the PWM control circuit 216 such that the motor current Im supplied to each of the inverter circuits 222 and the motor windings 126U, 126V, and 126W does not exceed a fixed upper limit value.
Then, based on the comparison result from the comparator 212, the PWM control circuit 216 performs pulse width control (PWM control) on the drive signal.
In this construction, the relationship between the motor current Im supplied to the inverter circuits 222 and the motor windings 126U, 126V, and 126W and the rotating speed ω of the rotor shaft 113 is as expressed by Equation 1.
                    E        =                              L            ⁡                          (                              ΔIm                                  Δ                  ⁢                                                                          ⁢                  t                                            )                                +                      R            ·            Im                    +                      K            ⁢                                                  ⁢            ω                                              Equation        ⁢                                  ⁢        1            
In Equation 1, L and R are the inductance component and the resistance component, respectively, of the motor windings 126U, 126V, and 126W, and K is the counter electromotive force constant of the motor windings 126U, 126V, and 126W; E is the drive voltage supplied from the inverter circuits 222.
Further, the relationship of Equation 1 is also apparent from the equivalent circuit for the inverter circuits and the motor windings shown in FIG. 10. In FIG. 10, a power source 251 corresponds to the drive voltage E supplied from the inverter circuits 222. An inductance 252 and a resistor 253 respectively correspond to the inductance component L and the resistance component R of the motor windings 126U, 126V, and 126W. Further, an AC power source 254 corresponds to the counter electromotive force Kω generated in the motor windings 126U, 126V, and 126W with the rotation of the rotor shaft 113.
The drive voltage E supplied from the inverter circuits 222 is of a fixed value, and the inductance component L, the resistance component R, and the counter electromotive force constant K of the motor windings 126U, 126V, and 126W are also values peculiar to the motor 121.
Thus, the motor current Im that can be supplied to the inverter circuits 222 and the motor windings 126U, 126V, and 126W is theoretically in conformity with Equation 1. In particular, the magnitude of the motor current Im when the rotating speed ω of the rotor shaft 113 is a rated RPM is referred to as the rated rotation current value Ir.
Here, when the rotor shaft 113 is to be rotated at the start of the turbo molecular pump, the RPM of the rotor shaft 113 and the reference RPM indicated by the command signal are compared with each other in the comparator 212, and PID compensation is effected. Further, a current command value, which is the output result thereof, and the motor current Im flowing through the motor windings 126U, 126V, and 126W are compared with each other to effect PID compensation.
Then, upon receiving this output result, the PWM control circuit 216 outputs a PWM-controlled drive signal to the inverter circuits 222 in the motor driving circuit 220, whereby power is supplied to the motor windings 126U, 126V, and 126W from the motor driving circuit 220, and an AC voltage is generated in the motor windings 126U, 126V, and 126W. Further, a torque corresponding to the motor current Im at this time is generated in the rotor shaft 113.
At the start of the turbo molecular pump, a large current value is generated as the current command value in the comparator 212; however, the comparator 212 is equipped with a current limiter circuit, with the motor current Im supplied to the inverter circuits 222 and the motor windings 126U, 126V, and 126W being controlled so as not to exceed an upper limit value set in this current limiter circuit. The upper limit value set in this current limiter circuit is the above-mentioned rated rotation current value Ir.
In this regard, at the start of the turbo molecular pump, the RPM of the rotor shaft 113 is less than the rated RPM, so that the counter electromotive force Kω is also less than the counter electromotive force Kω at rated rotation. Thus, at the start of the turbo molecular pump, according to the relationship of Equation 1, it is theoretically possible to supply a current value of not less than the rated rotation current value Ir, with the motor current being Im. However, to operate the inverter circuits 222 and the motor windings 126U, 126V, and 126W reliably and safely in the range of the RPM of the rotor 113 from zero to the rated RPM, there is set, as the upper limit value set in the current limiter circuit, the rated rotation current value Ir, which is the motor current Im when the rotor shaft 113 is at rated RPM, to allow a leeway in terms of safety.
Here, FIG. 11A shows the relationship between the starting time of the turbo molecular pump and the RPM of the rotor shaft 113, and FIG. 11B shows the relationship between the RPM of the rotor shaft 113 at this time and the motor current Im. The rated RPM of the rotor shaft 113 is approximately 37000 (rpm).
In FIG. 11A, the turbo molecular pump is started at time 0, and approximately 10 minutes thereafter, the RPM of the rotor shaft 113 attains the rated RPM. The time it takes the RPM of the rotor shaft 113 to reach the rated RPM is referred to as the starting time of the turbo molecular pump.
In FIG. 11B, from immediately after the starting of the turbo molecular pump until the RPM of the rotor shaft 113 reaches a level near the rated RPM, the motor current Im remains fixed. This is due to the fact that the motor current Im supplied to the inverter circuits 222 and the motor windings 126U, 126V, and 126W is controlled so as to be within a range not exceeding the upper limit value (the rated rotation current value Ir) set in the current limiter circuit.
Thereafter, when the RPM of the rotor shaft 113 reaches a level near the rated RPM, the motor current Im is reduced in accordance with a signal that has under gone PID compensation. During this period, the drive torque imparted to the rotor shaft 113 may be small, so that the quantity of electricity supplied from the motor driving circuit 220 to the motor windings 126U, 126V, and 126W is reduced or the supply is stopped, with the result that the motor current Im is smaller than that immediately after the starting of the turbo molecular pump.
Incidentally, in the conventional motor control system 300, at the start of the turbo molecular pump, a current value Ir at rated rotation is set as the upper limit of the motor current Im thereof, so that no torque in excess of the current value Ir at rated rotation is imparted to the rotor shaft 113, and the starting of the turbo molecular pump cannot be said to be quick enough.
The shipment of the turbo molecular pump involves a process in which the starting and stopping of the turbo molecular pump is repeated in order to inspect and evaluate the same; since the starting of the turbo molecular pump is slow, there is a fear of the production period for the turbo molecular pump becoming rather long.
Further, also when, in a case in which the turbo molecular pump is used for a movable simple vacuum chamber, the turbo molecular pump is re-started, due to its slow starting, there is a fear of the vacuum degree in the chamber being reduced, resulting in a reduction in the service life of the material and measurement filament.
Further, from now on, a flat panel display will tend to increase in size, and the turbo molecular pump will also tend to increase in size in order to improve the vacuum performance of the manufacturing apparatus for such a large flat panel display. Thus, in a turbo molecular pump equipped with the conventional motor control system 300, there is a fear of its starting becoming still slower.